In the field of audio instruments, high quality is required for signals. Accordingly, operations with high accuracy which do not cause even a small conversion error of an analog output signal are required for a digital-analog converter used for the audio instruments.
A digital-analog converter is a device that converts an input digital signal (input digital signal) into an analog signal and that outputs the converted analog signal as an analog output signal. In the digital-analog converter, a capacitive element is charged depending on a signal level of a digital input signal and an operational amplifier outputs an analog output signal depending on the charged voltage of the capacitive element. In the digital-analog converter having this configuration, a known technique for reducing a signal distortion is disclosed, for example, in Patent Document 1.
FIG. 15 is a diagram illustrating the digital-analog converter disclosed in Patent Document 1 and shows a configuration in which input signals VDin1, VDin2, . . . , VDinN (where N is a natural number in the present specification) corresponding to digital data are input and an analog output signal VAout is output. The configuration shown in FIG. 15 includes capacitive elements C1, C2, . . . , CN connected to input terminals D1, D2, . . . , DN of the input signals corresponding to plural digital data pieces via switches S11, S12, . . . , S1N. The plural capacitive elements C1, C2, . . . , CN are connected to switches 2 and 3. The switch 3 is connected to an inverting input terminal of an operational amplifier 5. In the operational amplifier 5, the inverting input terminal is connected to an output terminal A via a capacitive element 6.
The capacitive elements C1, C2, . . . , CN shown in FIG. 15 are sampling capacitive elements and the capacitive element 6 is an integrating capacitive element.
In the configuration shown in FIG. 15, nodes between the switches S11, S12, . . . , S1N and the capacitive elements C1, C2, . . . , CN are connected to the output terminal A of the operational amplifier 5. Switches S21, S22, . . . , S2N are disposed between the nodes between the switches S11, S12, . . . , S1N and the capacitive elements C1, C2, . . . , CN and the output terminal of the operational amplifier 5.
In a period in which control signals (not shown) input to the switch unit SWu1 and the switch 2 are at a “H” level, the capacitive elements C1, C2, . . . , CN are charged with capacities corresponding to signal levels of the digital input signals. After the switch unit SWu1 and the switch 2 are turned off, control signals input to the switch 3 and the switch unit SWu4 are changed to the “H” level . In this period, the capacitive elements C1, C2, . . . , CN and the capacitive element 6 are connected in series to each other and the capacitive elements C1, C2, . . . , CN and the output terminal A of the operational amplifier are connected. As a result, the potential of the output terminal A varies. The control signals input to the switch unit SWu1 and the switch 2 and the control signals input to the switch 3 and the switch unit SWu4 periodically vary so as to alternately repeat “H” and “L”.
(a) of FIG. 16 is a diagram illustrating a state where the output terminal A is connected between the switch unit SWu1 and the capacitive elements C1, C2, . . . , CN in a period in which the switch 3 and the switch unit SWu4 are turned on. Here, a resistance value (on-resistance value) when the switch unit SWu4 shown in FIG. 15 is turned on is defined as RSW4. The on-resistance value of the switch 3 shown in FIG. 15 is defined as RSW3. The capacitive elements C1, C2, . . . , CN shown in FIG. 15 are collectively referred to as a sampling capacitive element unit 7.
(b) of FIG. 16 is a diagram illustrating the waveform of an analog output signal output from the output terminal shown in (a) of FIG. 16. In (b) of FIG. 16, the vertical axis represents the level of an analog signal and the horizontal axis represents the time. (c) of FIG. 16 is a partially-enlarged view illustrating the waveform of the analog output signal shown in (b) of FIG. 16.
In the digital-analog converter, MOS transistors are generally used as the switches. The on-resistance value RSW3 of the switch 3 does not vary depending on the potential of the output terminal A. However, the on-resistance value RSW4 of the MOS transistor of the switch unit SWu4 varies depending on the potential of the output terminal A which is the source or drain terminal of the MOS transistor.